To setup SMTP authentication on outgoing mail server, please follow the steps below:
1. Start Mail.
2. From the Menu Bar, choose Mail -> Preferences -> The Accounts dialog box appears.
3. Click the Accounts icon in the top left corner of this dialog box.
4. Select OpenMail (or the name of your e-mail account) from the list of accounts on the left side of this dialog box.
5. Click the Server Settings button. The SMTP Server Options dialog box appears
6. In the Outgoing Mail Server field, type : mail.domain_name
7. In the Server port: 25/587.
8. To the right of Authentication:, choose Password from the drop-down list.
9. In the User Name: field, type your full Username which is your email address(firstname.lastname@example.org).
10. Please type your password on the Password field.
11. Click the OK button.
12. You will return to the Accounts dialog box. Close it.
13. Shut down and restart Mail